Method of and apparatus for recovery and utilization of sulphur dioxide in sulphite pulp manufacture



March 18, 1941. E M FRANKEL 2,235,759 METHOD OF AND APPARATUS FORRECOVERY AND UTILIZATION OF SULPHUR DIOXIDE IN SULPHITE PULP MANUFACTUREOriginal Filed Aug. 8, 1935 ATTORNEY Patented Mar. 18, 1941 UNITEDSTATES PATENT OFFICE 'i METHOD OF AND APPARATUS FOR RECOV- ERY ANDUTILIZATION F SULPHUR DI- OXIDE IN SULPHITE PULP MANUFACTURE Edward M.Frankel, Cedarhurst, Md., assignor to West Virginia Pulp and PaperCompany, New York, N. Y., a corporation of New York Application August8, 1935, Serial No. $5,289

Renewed April 25, 1940 7 Claims.

My present invention relates to improvements in the recovery andutilization of sulphur dioxide and other values in sulphite pulpmanufacture.

While the advantages of cooking with high concentrations of SO2, namely,better ber penetration, shorter cooking time at lower temperatures,resulting in stronger pulp and higher yields, have been known, yet suchadvantages have been imperfectly realized in practice. It is accordinglya principal object of my invention to devise an improved recoveryprocess and apparatus while providing for an easily controlledcirculation of SO2 through the digester and recovery system whereby theconcentration of SO2 in the digester may be kept at the highest valueconsistent with the temperature of cooking selected. A further object isto provide such a process and apparatus which will be characterized bysimplicity of construction and reliability of operation,

My invention is predicated essentially upon an improved handling of thedigester relief. In the operation of sulphite pulp digesters, Wood chipsare placed in the digester and covered with sulphite liquor containingfree sulphur dioxide. The sealed digester is then brought up totemperature by passing steam thereinto. As the digester heats up, theliquid expands and sulphur dioxide, air, and other gases are released.It is therefore necessary to allow for this expansion in volume byventing the top of the ldigester and carrying away the mixture of liquidand gases constituting the so-called relief. i

A number of schemes have been proposed for an efncient disposition ofthe relief but none of these has been entirely successful. It has beenattempted to recover the sulphur dioxide by subjecting the gaseouscomponent of the relief directly to condensation, but because of thepresence of the non-condensables, i. e. air, CO2, etc., the recovery ofthe SO2 by cooling means ordinarily available is very incomp1ete. It hasalso been proposed to recover values in the relief by bringing therelief into contact with a stream of cold acid in an injector device,the velocity of the stream of acid being sunicient to inject the entirequantity of relief into an accumulator. This, however, is subject to thedrawback that the non-condensable gases are carried along and arethereby introduced into the accumulator. The presence of suchnon-condensables in the relief accumulator is a distinct `disadvantagebecause they collect above the acid, thereby diluting the sulphurdioxide in the vapor space and reducing the effective `pressure forholding the sulphur dioxide in the acid. It is then necessary to have arelatively low sulphur dioxide concentration or low temperature, orboth, in the acid stored.

I have now discovered that greatly improved 5 results are to be had bysubjecting the gaseous component of the relief (the relief beingseparated into a liquid component and a gaseous component as heretoforeproposed) to absorption i preferably by cold raw acid liquor under pres-10 sure, thereby capturing the SO2 while permitting thenon-condensables, such as air, CO2, etc., to be vented from the system,after which all or a portion of the SO2 absorbed by the liquor may beliberated by heating and liquefied by simple l5 condensation.Preferably, both the liquid component of the relief and the SO2 chargedacid liquor are passed to an accumulator from which the SO2 may beliberated and condensed as just stated. The SO2 so condensed is then ledto the 20 SO2 accumulator to be made available for treatment of thecharge in the same or the next cook.

Such a system has very distinct advantages, among which may bementioned:

(1) Practically complete recovery of the sul- 25 phur dioxide and heatresults while at the same time the non-condensable gases are vented fromthe system. It will be noted that since the bulk of the heat in therelief is held by the liquid and since there is no reduction of pressurein the ab- 30 sorber system, the absorber acid is warmed comparativelylittle whereby its absorption eliciency is maintained at a high level.In prior recovery systems, a large amount of relatively strong SO2 isvented to the raw acid system at low pressure under conditions resultingin a low efficiency of recovery.

(2) It will be further noted that the recovery of the pure sulphurdioxide is entirely independent of conditions in the accumulator andthat therefore the temperature of the latter may be maintained at anydesired value. If the acid charged to the accumulator is notsufiiciently fortified with sulphur dioxide, then it may be surchargedwith SO2 from the SO2 accumulator. If 45 because of the conditions ofthe system, the acid in the accumulator is saturated with the sulphurdioxide, any further accumulation of SO2 carried in by the absorptionliquor above saturation is given off and caught in the condensercommunieating with the SO2 accumulator. Due to the absence of thenon-condensable gases, the amount of SO2 held by the accumulator acid isstrictly a function of the temperature and the pressure prevailingtherein. Since it is comparatively easy to keep these values constant,the SO2 content of the acid is therefore uniform thereby promotinguniform conditions as to the SO2 content when the accumulator acid isused in the next cook.

(3) Because of the ease of eliminating from the system thenon-condensable gases and of the circulation of SO2 through the system,my improved system readily lends itself to a method of cookingheretofore proposed wherein the chips are rst exhausted of theircontained air preferably by the action of a stream of sulphur d1- oxidepassed through the chips before charging with acid. This elimination ofair increases the rate of penetration of the cooking acid and thusreduces the tendency of the chips to burn at high heating rates.

(4) A further and highly important advantage is that the improved methodlends itself to a novel method of cooking in which the liquor charged onthe chips carries suilicient heat to bring the whole mass to cookingtemperature without the further addition of steam to the digester butwith addition of SO2 as needed. Preferably the chips are previouslyheated with steam and the air displaced either by steam or SO2.

Other advantages will be apparent as the description proceeds and thefeatures of novelty will lbe pointed out in the claims. For the bestunderstanding of the invention, reference may now be had to thefollowing detailed description taken with the annexed drawing showingdiagrammatically an illustrative embodiment.

As shown, a digester I0 is provided with the usual steam admission pipeII, blow pipe I2 and relief pipe I3 (which pipe may also receive relieffrom othei` digesters not shown), the digester being also provided witha constant pressure relief valve I5 of known construction connected withthe digester by means of a pipe IB. Relief pipe I3 communicates with atrap I8 which acts to separate the relief into a liquid portion whichpasses downwardly through a pipe I9 to an accumulator 2U, and a gaseousportion which passes upwardly through a pipe 22 to an absorber 23. Theabsorber 23, which desirably is of the plate type, has a feed pipe 24for the absorption menstruum under pressure and a relief pipe 25 whichhas an automatic valve 28 by means of which the column 23 is vented ofthe non-condensable gases. The SO2 enriched liquor travels downwardlyfrom the column 23 to a pipe 28 having preferably a U bend acting as aliquid seal to the top of accumulator 20.

To take care of SO2 and steam released from the accumulator 20, there isprovided a pipe 3| leading to an absorber 33, pipe 34 leading from thetop of same to a condenser 35 having a cooling coil 36 and a reux returnpipe 3'I which has a U bend. The function of the absorber-condenser unit33-35 is to condense mostly water vapor in the absorber 33 by action ofthe refiux from condenser 35 and to condense the SO2 in the condenser35. The water condensed in column 33 is returned to the accumulator bymeans of the pipe 38 also having a U bend. Pipe 40 conveys the liquidSO2 to trap 4I where any vapors or non-condensable gases are separatedand pass upwardly through pipe 42 to absorber 23, said pipe also actingas a pressure equalizer for the system. The liquid SO2 passes downwardlyfrom trap 4I through pipe 43 to the SO2 accumulator 44. The liquid SO2is withdrawn from accumulator 44 as required through pipe 46, pump 4l(which latter may be a metering device) and pipe 48 by means of which itmay be passed either to digester I0 or some other digester in the seriesnot shown, it being understood that a battery of several digesters willbe operated with a single accumulator 20.v

The operation of my improved system in carrying out a cook with steamand SO2 may be as follows: Chips are charged into the digester I3 andafter putting on the cover acid is pumped into the digester from theaccumulator 20 by means of pump 52 and pipe 53. If desired, the acid maybe pumped through the charge and back to the accumulator through pipe I9for a period of time. After the digester is charged with liquor, it maybe fortified with sulphur dioxide by pumping the liquid sulphur dioxideinto the bottom of the digester from accumulator 44 by means of pump ormetering device 4l. After suiicient sulphur dioxide is injected, steamis introduced into the digester through pipe II until thev desiredtemperature is reached or according to any other schedule. The pressurein digester I0 is regulated by valve I5 which is set to hold thepressure constant within a safe limit of the vessel. As the pressure inthe digester increases the valve I5 opens to let out the socalledrelief. The latter is separated in the trap i8 into a liquid componentwhich passes to the accumulator 20 by pipe I9 as previously describedwhile the gas passes to absorber 23 by means of pipe 22, the pressure onthe accumulator and liquid sulphur dioxide storage being regulated bythe control vent. valve 26. The menstruum entering pipe 24 underpressure, which is desirably the cold raw treating liquor, becomes ladenwith sulphur dioxide and then passes through the trapped pipe 28 to theaccumulator 20. If the temperature of the mixture in the accumulator ishigh enough, a mixture of sulphur dioxide and water vapor is evolved andthese are conveyed to the fractionating column 33 where the water isremoved and returned preferably to the accumulator by pipe 38 aspreviously mentioned. The sulphur dioxide is condensed in the condenser35 and conveyed to SO2 accumulator 44, a small amount being used asreflux through pipe 3l. Any uncondensed gas is conveyed through pipe 42to absorber 23 where it is mixed with the vapors arising from therelief.

The composition of the liquor fed to the absorber will naturally be thatwhich it is desired to use in the subsequent cook, i. e. calcium,magnesium, ammonium, or sodium sulphite, or mixtures of these, togetherwith some dissolved SO2 but not. enough to prevent the liquor fromacting as an absorbent for the SO2 in the relief. It is not necessarythat the liquor be acid to start with although if it is alkaline it willpick up CO2 from the gases which is not desirable. The pressure shouldbe about that of the digester, a typical value of which is around 60 to75 lbs. gauge so that the pressure within the accumulator may be justslightly under that of the digester.

As previously stated, the cooking operation may be carried out in what.I believe to be a novel manner by first charging the digester withchips, removing the air therefrom as by evacuation or by displacementwith sulphur dioxide or steam or both, after which thek chips are warmedto say -90 degrees C. with open steam, then the liquor from theaccumulator 20,' say at 140 degrees C., is applied thereby bringing upthe digester contents to around 120 degrees C. because of the largeratio of liquor to chips used. Then having the digester at the desiredtemperature, sulphur dioxide is circulated through the digester untilthe cook is complete, after which the digester can be blown. Desirably,the sulphur dioxide in the blow liquor will be recovered by known means.

Various changes may be made in the process and apparatus withoutdeparting from the spirit of the invention. In the claims, by acidliquor I mean liquor containing soluble sulphites suitable for digestionof wood whether such liquor be actually acid or not.

I claim:

1. In a sulphite digesting system, the method of recovering digesterrelief, which consists in rst separating said relief into a liquid and agaseous component but without substantial cooling of said relief,separating the SO2 from the non-condensibles in said gaseous componentby subjecting said gaseous component to the absorptive action underpressure of a non-alkaline menstruum of a nature as to constitute acomponent of the cooking liquor for the same or subsequent oooks,thereby dissolving said SO2 to the substantial exclusion of thenon-condensibles, Venting the latter from the system, conducting theSC2-enriched menstruum to an acid accumulator, likewise under pressure,continuing the admission of said menstruum to said accumulator whilemaintaining the temperature of the latter suciently high to release anexcess of SO2 gas, withdrawing said excess SO2 from said accumulator,and subjecting the SO2 to cooling to liquefy same.

2. The method according to claim 1 in which said accumulator i-smaintained in equilibrium with said gaseous component undergoingabsorption and said excess SO2 gas undergoing cooling to liquefaction.

3. In a sulphite digesting system, the method of recovering digesterrelief, which consists in first separating said relief into a liquid anda gaseous component but without substantial cooling of said relief,separating the SO2 from the non-condensibles in said gaseous componentby subjecting said gaseous component to the absorptive action underpressure of a non-alkaline menstruum of a nature as to constitute acomponent of the cooking liquor for the same or subsequent cooks, saidmenstruum being fed in counter current relationship to said gaseouscomponent, thereby dissolving said SO2 to the substantial exclusion ofthe non-condensibles, venting the latter from the system, conducting theSO2- enriched menstruum to an acid accumulator, likewise under pressure,continuing the admission of said menstruum to said accumulator whilemaintaining the temperature of the latter suciently high to release anexcess of SO2 gas, withdrawing said excess SO2 from said accumulator,separating said SO2 from attendant water vapor by rectification, andsubjecting the `SO2 to cooling to liquefy same.

4. In a sulphite digesting system, the method of recovering digesterrelief, which consists in rst separating said relief into a liquid and agaseous component but without -substantial coolu ing of said relief,separating the SO2 from the non-condensibles in said gaseous componentby subjecting said gaseous component to the absorptive action underpressure of a non-alkaline menstruum of a. nature as to constitute acomponent of the cooking liquor for the same or subsequent cooks,thereby dissolving said SO2 to the substantial exclusion of thenon-condensibles, venting the latter from the system, conducting theSO2-enriched menstruum to an acid accumulator likewise under pressure,continuing the admission of said menstruum to said accumulator whilemaintaining the temperature of the latter sufficiently high to releasean excess of SO2 gas, withdrawing said excess SO2l from saidaccumulator, separating said SO2 from attendant water vapor byrectication, and subjecting the SO2 to cooling to liquey same, whileventing non-condensibles at this point.

5. The method according to claim 1, including the step of adding theliquid component of the relief to said accumulator.

6. The method according to claim 1 in which the pressures employed inthe SO2 absorption step and in the acid accumulator are approximatelythat of the digester.

7. In a sulphite digesting system, the method of recovering digesterrelief, which consists in first separating said relief into a liquid anda gaseous component but without substantial cooling of said relief,separating the SO2 from the non-condensibles in said gaseous componentby subjecting said gaseous component to the ab- -sorptive action underpressure of a non-alkaline menstruum of a nature as to constitute acomponent of the cooking liquor for the same or subsequent cooks,thereby dissolving said SO2 to the substantial exclusion of thenon-condensibles, venting the latter from the system, conducting theSO2-enriched menstruum to an acid accumulator, likewise under pressure,continuing the admission of said menstruum to said accumulator whilemaintaining the temperature of the latter sufliciently high to releasean excess of SO2 gas, withdrawing said excess SO2 from said accumulator,and subjecting the SO2 to cooling to liquefy same and passing liquefiedSO2 so obtained back to the digester.

EDWARD M. FRANKEL.

